1. Field of the Invention
The present invention generally relates to an ocular examination system for examining the blood vessels of an eye fundus.
2. Description of the Related Art
The principle of a measurement operation performed by a retinal blood flow meter, which is an example of an ocular examination system, is discussed below. A laser beam having a wavelength xcex is applied to a subject blood vessel of a patient""s eye, and the light scattered and reflected by the blood vessel is received by a photodetector. Then, an interference signal of a Doppler shift component, i.e., the light scattered and reflected by the blood moving in a blood vessel and the light scattered and reflected by a stationary blood vessel wall is detected. Upon analyzing the frequency of the interference signal, the blood velocity is determined. More specifically, the blood velocity, i.e., the maximum Vmax, is determined according to the following equation:
xe2x80x83Vmax={xcex/(nxc2x7xcex1)}xc2x7||xcex94fmax1|xe2x88x92|xcex94fmax2||/cos xcex2xe2x80x83xe2x80x83(1)
wherein xcex94fmax1 and xcex94fmax2 indicate the maximum frequency shifts calculated from the received-light signals received by two photodetectors; xcex represents the wavelength of the laser light; n designates the index of refraction of a portion of the eye to be examined; a indicates the angle between the two light-detecting optical axes within the eye; and xcex2 represents the angle between the plane formed by the two light-detecting optical axes and the velocity vector of the blood flow.
By measuring the blood velocity from the two directions as discussed above, contributions due to the directions of incidence of the measuring beams are canceled, thereby making it possible to measure the velocity of blood at a certain portion on the eye fundus. By matching the line of intersection between the plane formed by the two light-detecting optical axes and the eye fundus to the angle xcex2, xcex2 becomes 0 degrees, thereby measuring the true maximum velocity.
According to the retinal blood flow meter for measuring the shape of a blood vessel or the blood velocity in a blood vessel in a particular portion of the eye fundus by utilizing the laser beam, it is necessary that a beam of measuring light is precisely applied to a subject portion for a predetermined period. In actuality, however, it is difficult to precisely keep applying the measuring light to the subject portion due to involuntary eye movement. In order to solve this problem, an ocular system provided with a tracking function for detecting the position of a blood vessel and moving the measuring light to the subject portion in accordance with involuntary eye movement in real time is disclosed in Japanese Patent Laid-open Nos. 63-288133 and 6-503733 (by PCT).
In the above-described ocular system, the tracking operation is performed as follows. A linear charge-coupled device (CCD) is used for applying a beam of tracking light to an eye fundus and for receiving light reflected by the eye fundus and producing a blood-vessel-image signal. Then, the waveform of the blood-vessel-image signal is processed so as to calculate the amount of movement of the blood-vessel image from a tracking reference position. Tracking light emitted from a tracking illumination light source and measuring light are applied to an eye fundus via a pupil conjugate mirror, and the linear CCD is used for receiving the light reflected by the eye fundus so as to process the waveform of the blood-vessel-image signal. A system for calculating a blood-vessel diameter by using a blood-vessel-image signal output from a linear CCD is disclosed in Japanese Patent Laid-open No. 7-31596.
However, the above-described ocular examination systems have to perform a tracking operation while following the fast eye movement, and thus, a sufficient accumulation time of the linear CCD cannot be ensured. Accordingly, in order to obtain a blood-vessel image having a sufficient amplitude for performing the tracking operation and for calculating the blood-vessel diameter, an image intensifier is used for performing light amplification. However, with an excessively high amplification factor, the signal-to-noise (S/N) ratio of the blood-vessel-image signal from the linear CCD is reduced, thereby resulting in the lowered measurement precision of the blood-vessel diameter.
This problem may be solved by removing unwanted high-frequency noise components by performing filtering processing. However, since various diameters of blood vessels are measured, the feature points for calculating the blood-vessel diameter may inconveniently be removed together with the noise components depending on the setting of the cut-off frequency. Alternatively, the intensity of tracking light applied to a patient""s eye fundus may be uniformly increased so as to reduce the amplification factor of the image intensifier, thereby suppressing noise components. In the aforementioned ocular examination systems, however, the maximum permissible exposure (MPE), which is the maximum permissible laser energy to be applied to a patient""s eye, is set by the American National Standard Institute (ANSI) for the safety of the patients. Accordingly, measuring light is applied to a patient""s eye without exceeding the MPE, resulting in a limitation on the number of measurements for the same subject portion of the eye.
Accordingly, a major object of the present invention is to improve a conventional ocular examination system. More specifically, it is an object of the present invention to achieve high-precision ocular examination by applying light to an eye fundus at a suitable intensity level.
It is another object of the present invention to achieve high-precision ocular examination without restricting the number of measurements for the same subject portion of the eye.
In order to achieve the above objects, according to the present invention, there is provided an ocular examination system comprising an illumination system configured to illuminate a region of an eye fundus of an eye including a target blood vessel, the illumination system being configured to adjust the intensity of light illuminating the region, an image pickup device positioned and configured to,receive light scattered from the region illuminated by the illumination system and to produce signals in response to receiving the scattered light from the region, a control system connected to the illumination system and the image pickup device so as to receive the signals produced by the image pickup device and configured to compute the diameter of the target blood vessel based on the signals from the image pickup device, and a tracking system connected to the control system and configured to perform an automatic tracking operation on the target blood vessel based on the signals from the image pickup device. The control system is configured to control the tracking system so that the tracking system performs the automatic tracking operation simultaneous with the control system computing the diameter of the target blood vessel and so that the tracking system performs the automatic tracking operation when the control system does not computer the diameter of the target blood vessel. The control system is also configured to control the illumination system to change the intensity of the illumination applied to the region when the automatic tracking operation and the target-blood-vessel-diameter computation are simultaneously performed above the level of the illumination applied to the region when the automatic tracking operation is performed while the control system does not compute the target-blood-vessel diameter.
According to another aspect, the present invention that achieves these objectives relates to an ocular examination system comprising illumination means for illuminating a region of an eye fundus of an eye including a target blood vessel and for adjusting the intensity of light illuminating the region, image pickup means for receiving light scattered from the region illuminated by the illumination means and for producing signals in response to receiving the scattered light from the region, control means for computing the diameter of the target blood vessel based on the signals from the image pickup means, and tracking means for performing an automatic tracking operation on the target blood vessel based on the signals from the image pickup means. The control means comprises means for controlling the tracking means so that the tracking means performs the automatic tracking operation simultaneous with the control means computing the diameter of the target blood vessel and so that the tracking means performs the automatic tracking operation when the control means does not compute the diameter of the target blood vessel. The control means also comprises means for controlling the illumination means to change the intensity of the illumination applied to the region when the automatic tracking operation and the target-blood-vessel-diameter computation are simultaneously performed above the level of the illumination applied to the region when the automatic tracking operation is performed while the control means does not compute the target-blood-vessel diameter.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.